How to Calculate Subwoofer Box Volume
The size of your speaker cabinet dictates how deep and how loud your subwoofer will play. If your math is wrong, your bass will be muddy or nonexistent. Here is how to calculate your box volume correctly.
Gross Volume vs. Net Volume
Before doing any math, you must understand the difference between the two types of volume used in audio design. When a subwoofer manufacturer says a speaker needs "2.0 cubic feet," they are almost always referring to Net Volume.
Gross Volume
This is the total internal air space inside your wooden box before you put anything inside it. It is simply the internal Height × Width × Depth.
Net Volume
This is the actual usable air space that the subwoofer "sees." To get the net volume, you must take your Gross Volume and subtract the physical space taken up by the subwoofer magnet (driver displacement), the port tube, and any internal wooden bracing.
The Basic Math: Height × Width × Depth
To calculate the internal volume of a rectangular box, you multiply the internal dimensions. Remember to subtract the thickness of your wood (like 3/4" MDF) from the external dimensions to find the true internal size.
Calculating in Cubic Feet (cu. ft.)
If your internal measurements are in inches:
Volume (cu ft) = (Height × Width × Depth) ÷ 1728
Example: An internal box of 15" × 15" × 15" = 3375 cubic inches. Divide by 1728 = 1.95 cubic feet.
Calculating in Liters (L)
If your internal measurements are in centimeters:
Volume (Liters) = (Height × Width × Depth) ÷ 1000
Example: An internal box of 40cm × 40cm × 40cm = 64,000 cubic centimeters. Divide by 1000 = 64 Liters.
Skip the Math Errors
Calculating a simple cube is easy. But what happens when you need exactly 55 Liters of Net Volume, and you have to subtract 3.5 Liters for the subwoofer motor, 8 Liters for a slot port, and account for 18mm MDF thickness? The math gets complicated fast, and a small error means your plywood cut sheet will be ruined.
Use the Speaker Box Calculator
Stop guessing and use our free subwoofer enclosure designer. Just enter your target volume, wood thickness, and driver displacement. We will instantly generate your exact 3D assembly models and give you the perfect external panel dimensions to send straight to the saw.
Calculate My Box